fdt_region: Check return value of fdt_get_property_by_offset() calls

fdt_region: Check return value of fdt_get_property_by_offset() calls

fdt_get_property_by_offset() returns NULL for FDT with version
less than 0x10. fdt_find_regions() dereferences the result without
checking, leading to a NULL pointer dereference during signature
verification of an untrusted FIT. fdt_add_alias_regions() and
fdt_next_region() also lack validation.

Add NULL checks before accessing the returned property pointer.
Also add a missing NULL check for fdt_string() in
fdt_add_alias_regions() and fdt_next_region().

Signed-off-by: Anton Ivanov <anton@binarly.io>